Galvanic oxygen sensors are widely applied in high volumes in industrial, environmental and medical measurements because of their high reliability, small size, low power consumption and good price performance relation. Starting with their development in the 1950's many instruments to measure oxygen partial pressure were designed, some of them still being used today. In modem instruments like ventilators, emission monitoring devices and automotive emission testers such sensors are still preferred for oxygen measurements.
Typically, the sensors comprise a housing, a cathode, an anode having a bigger surface than the cathode, a diffusion barrier, an electrolyte and contact wires to connect the cathode and the anode electrically. The anode generates the required electrochemical potential for the reduction of oxygen at the cathode. Such galvanic sensors are described in U.S. Pat. No. 3,767,552 and U.S. Pat. No. 3,429,796.
GB 1255353 describes a galvanic oxygen sensor with an anode material being made of lead, tin, cooper and their alloys. The electrolyte of this sensor contains sulfides. This design leads to very stable sensor signals that can be amplified electronically very well. On the other hand it is a considerable drawback of this design that the sensor can not be used in acid gas atmospheres containing e.g. carbon dioxide. In this case poisonous sulfur dioxide is can be released. Therefore the sensor can not be used for medical applications.
GB 1391168 describes a device for the measurement of oxygen. This device has two oxygen permeable membranes, one of them being porous, a silver cathode, and a tubular tin anode. This configuration allows the device to measure in condensing media or atmospheres that contain water droplets like in humidifiers for breath. The porous membrane prevents, due to its hydrophobic properties, the formation of a closed water film on the surface which could cause a signal reduction. The composition of the electrolyte is not described in the application. The principle to use a combination of two membranes, one of them being porous and thus preventing water condensation is state of the art for oxygen sensors today.
EP 1 593 962 describes a lead free galvanic oxygen sensor with an anode being made of zinc or aluminum. The inventors of this '962 patent document admit that these materials corrode and thus are only stable within a narrow pH-range of the electrolyte. Such sensors have a very limited lifetime when they are used at elevated temperatures because the corrosion is very temperature dependent. Additionally, the corrosion process generates water that has to be removed from the sensor. This can lead to a complex design of the sensor housing.
For a galvanic oxygen sensor with electrolyte and anode, the equations for the electro-chemical reactions are as follows.
Equation for the electrochemical process at the cathode with an alkaline electrolyte:O2+2H2O+4e−→4OH−  (Eq. 1a)
In an acid electrolyte the reduction of oxygen consumes protons:O2+4H++4e−→2H2O   (Eq. 1b)
Equations for possible electrochemical processes at the anode depending on the actual pH at the anode surface:Me+4OH−→MeO2+4e−+2H2O   (Eq. 2a)2Me→2Me2++4e−  (Eq. 2b)
Me stands for any metal that may exist at bivalent or tetravalent oxidation stage after the oxidation. According to equation 2a, the oxidation of the anode material leads to an oxide. According to equation 2b, the oxidation of the anode material leads to a soluble salt.
In EP 0305961, buffered weak acid electrolytes are described that consist of organic acids and their salts leading to oxygen sensors with increased lifetime.
The diffusion membrane in a sensor is a barrier for the gas so that a diffusion limiting current is generated at the cathode that can be measured. The diffusion limited current is proportional to the gas partial pressure at the diffusion barrier by the first approximation.
Instead of a membrane, a metal or plastic disc with a very small central hole can also be used. Given the appropriate dimension of the hole, the diffusion of the oxygen happens according to the Knudsen principle. This also leads to a diffusion limited current as long as all oxygen molecules reaching the cathode are being reduced. In this case, the sensor signal is proportional to the oxygen concentration at the sensor head. A device based on this principle is described in EP 0763730.
The electrical current flowing through the galvanic cell depends linearly on the oxygen partial pressure or the oxygen concentration. The electrochemically active surface of the anode should be bigger than the surface of the cathode to ensure an adequate motive force for the reaction and to avoid a concentration polarization at the surface of the anode.
In practice, lead is the state of the art anode material. It has a high hydrogen overvoltage and is thus corrosion resistant in alkaline and weak acid electrolytes over a wide temperature range. The relative high density of the lead permits small designs of the anode. In addition, it can be shaped easily due to its softness and is available in high purity at a relatively low price.
However, for some years lead has been disliked as a construction material. It is known that relatively low chronic doses are harmful to the human nervous system, the hematopoietic system and the kidneys. Thus, the limits for lead and lead containing substances in the environment have been repeatedly lowered in the past years. New legislation in various countries and states prohibits the use of lead in solders and electronic components. Also other heavy metals like cadmium and mercury are banned by the European Union's RoHS (Restriction of Hazardous Substances), and as such they are not viable alternatives to be used in electrochemical oxygen sensors.
As such, there is an need for a lead free, galvanic oxygen sensor.